Fuel cell electrocatalyst of pt-mn-co

ABSTRACT

An improved metal alloy composition for a fuel call catalyst containing platinum, manganese, and cobalt.

BACKGROUND OF THE INVENTION

1. Field of The Invention

The present invention relates to metal catalysts, especially to ternarycatalysts which comprise platinum, manganese and cobalt, which areuseful in fuel cell electrodes and other catalytic structures.

2. Background Information

A fuel cell is an electrochemical device for directly converting thechemical energy generated from an oxidation-reduction reaction of a fuelsuch as hydrogen or hydrocarbon-based fuels and an oxidizer such asoxygen gas (in air) supplied thereto into a low-voltage direct current.Thus, fuel cells chemically combine the molecules of a fuel and anoxidizer without burning, dispensing with the inefficiencies andpollution of traditional combustion.

A fuel cell is generally comprised of a fuel electrode (anode), anoxidizer electrode (cathode), an electrolyte interposed between theelectrodes (alkaline or acidic), and means for separately supplying astream of fuel and a stream of oxidizer to the anode and the cathode,respectively. In operation, fuel supplied to the anode is oxidized,releasing electrons which are conducted via an external circuit to thecathode. At the cathode, the supplied electrons are consumed when theoxidizer is reduced. The current flowing through the external circuitcan be made to do useful work.

There are several types of fuel cells, including those havingelectrolytes of: phosphoric acid, molten carbonate, solid oxide,potassium hydroxide, and proton exchange membrane. A phosphoric acidfuel cell operates at about 160-220° C., and preferably at about 190-200° C. This type of fuel cell is currently being used for multi-megawattutility power generation and for co-generation systems (i.e., combinedheat and power generation) in the 50 to several hundred kilowatts range.

In contrast, proton exchange membrane fuel cells use a solidproton-conducting polymer membrane as the electrolyte. Typically, thepolymer membrane is maintained in a hydrated form during operation inorder to prevent loss of ionic conduction which limits the operationtemperature typically to between about 70 and about 120° C. depending onthe operating pressure, and preferably below about 100° C. Protonexchange membrane fuel cells have a much higher power density thanliquid electrolyte fuel cells (i.e., phosphoric acid), and can varyoutput quickly to meet shifts in power demand. Thus, they are suited forapplications such as in automobiles and small scale residential powergeneration where quick startup is a consideration.

In some applications (i.e., automotive) pure hydrogen gas is the optimumfuel; however, in other applications where a lower operational cost isdesirable, a reformed hydrogen-containing gas is an appropriate fuel. Areformed-hydrogen containing gas is produced, for example, bysteam-reforming methanol and water at 200-300° C. to a hydrogen-richfuel gas containing carbon dioxide. Theoretically, the reformate gasconsists of 75 vol % hydrogen and 25 vol % carbon dioxide. In practice,however, this gas also contains nitrogen, oxygen, and, depending on thedegree of purity, varying amounts of carbon monoxide (up to 1 vol %).Although some electronic devices also reform liquid fuel to hydrogen, insome applications the conversion of a liquid fuel directly intoelectricity is desirable, as then a high storage density and systemsimplicity are combined. In particular, methanol is an especiallydesirable fuel because it has a high energy density, a low cost, and isproduced from renewable resources.

For the oxidation and reduction reactions in a fuel cell to proceed atuseful rates, especially at operating temperatures below about 300° C.,electrocatalyst materials are typically provided at the electrodes.Initially, fuel cells used electrocatalysts made of a single metal,usually platinum (Pt), palladium (Pd), rhodium (Rh), iridium (Ir),osmium (Os), silver (Ag) or gold (Au) because they are able to withstandthe corrosive environment—platinum being the most efficient and stablesingle-metal electrocatalyst for fuel cells operating below about 300°C. While these elements were first used in fuel cells in metallic powderform, later techniques were developed to disperse these metals over thesurface of electrically conductive supports (i.e., carbon black) toincrease the surface area of the electrocatalyst which in turn increasedthe number of reactive sites leading to improved efficiency of the cell.Nevertheless, fuel cell performance typically declines over time becausethe presence of electrolyte, high temperatures and molecular oxygendissolve the electrocatalyst and/or sinter the dispersed electrocatalystby surface migration or dissolution/re-precipitation.

Although platinum is the most efficient and stable single-metalelectrocatalyst for fuel cells, it is costly and an increase inelectrocatalyst activity over platinum is necessary for wide scalecommercialization of fuel cell technology. The development of cathodefuel cell electrocatalyst materials faces longstanding challenges. Thegreatest challenge is the improvement of the electrode kinetics of theoxygen reduction reaction. In fact, sluggish electrochemical reactionkinetics have prevented attaining the thermodynamic reversible electrodepotential for oxygen reduction. This is reflected in exchange currentdensities of around 10⁻¹⁰ to 10⁻¹² A/cm² for oxygen reduction on, forexample, Pt at low and medium temperatures. A factor contributing tothis phenomenon include the fact that the desired reduction of oxygen towater is a four-electron transfer reaction and typically involvesbreaking a strong O—O bond early in the reaction. In addition, the opencircuit voltage is lowered from the thermodynamic potential for oxygenreduction due to the formation of peroxide and possible platinum oxideswhich inhibit the reaction. A second challenge is the stability of theoxygen electrode (cathode) during long-term operation. Specifically, afuel cell cathode operates in a regime in which even the most unreactivemetals are not completely stable. Thus, alloy compositions which containnon-noble metal elements may have a rate of corrosion which wouldnegatively impact the projected lifetime of a fuel cell. The corrosionmay be more severe when the cell is operating near open circuitconditions (which is the most desirable potential for thermodynamicefficiency).

Electrocatalyst materials at the anode also face challenges during fuelcell operation. Specifically, as the concentration of carbon monoxide(CO) rises above about 10 ppm in the fuel the surface of theelectrocatalyst can be rapidly poisoned. As a result, platinum (byitself is a poor electrocatalyst if the fuel stream contains carbonmonoxide (i.e., reformed-hydrogen gas typically exceeds 100 ppm). Liquidhydrocarbon-based fuels (e.g., methanol) present an even greaterpoisoning problem. Specifically, the surface of the platinum becomesblocked with the adsorbed intermediate, carbon monoxide (CO). It hasbeen reported that H₂O plays a key role in the removal of such poisoningspecies in accordance with the following reactions:Pt+CH₃OH→Pt—CO+4H⁺+4e⁻  (1);Pt+H₂O→Pt—OH+H⁺+e⁻  (2); andPt—CO+Pt—OH→2Pt+CO₂+H⁺+e⁻  (3).As indicated by the foregoing reactions, the methanol is adsorbed andpartially oxidized by platinum on the surface of the electrode (1).Adsorbed OH, from the hydrolysis of water, reacts with the adsorbed COto produce carbon dioxide and a proton (2,3). However, platinum does notform OH species well at the potentials fuel cell electrodes operate(e.g., 200 mV−1.5 V). As a result, step (3) is the slowest step in thesequence, limiting the rate of CO removal, thereby allowing poisoning ofthe electrocatalyst to occur. This applies in particular to a protonexchange membrane fuel cell which is especially sensitive to COpoisoning because of its low operating temperatures.

One technique for increasing electrocatalytic cathodic activity duringthe reduction of oxygen and electrocatalytic anodic activity during theoxidation of hydrogen is to employ an electrocatalyst which is moreactive, corrosion resistant, and/or more poison tolerant. For example,increased tolerance to CO has been reported by alloying platinum andruthenium at a 50:50 atomic ratio (see, D. Chu and S. Gillman, J.Electrochem. Soc. 1996, 143, 1685). The electrocatalysts proposed todate, however, leave room for further improvement.

BRIEF SUMMARY OF THE INVENTION

Briefly, therefore, the present invention is directed to a ternarycatalyst for use in oxidation or reduction reactions. The ternarycatalyst comprises platinum, manganese, and cobalt, with theconcentration of manganese being no greater than 20 atomic percent.

The present invention is further directed to such a catalyst, whereinsaid catalyst is an alloy.

The present invention is still further directed to a supportedelectrocatalyst powder for use in electrochemical reactor devices. Thesupported electrocatalyst powder comprises a ternary catalyst whichcomprises platinum, manganese, and cobalt, wherein the concentration ofmanganese is no greater than 20 atomic percent, and electricallyconductive support particles upon which the ternary catalyst isdispersed.

The present invention is still further directed to a fuel cell electrodewhich comprises electrocatalyst particles and an electrode substrateupon which the electrocatalyst particles are deposited. Theelectrocatalyst particles comprise a ternary catalyst which comprisesplatinum, manganese, and cobalt, wherein the concentration of manganeseis no greater than 20 atomic percent.

The present invention is still further directed to a fuel cell whichcomprises an anode, a cathode, a proton exchange membrane between theanode and the cathode, and a ternary catalyst which comprises platinum,manganese, and cobalt, wherein the concentration of manganese is nogreater than 20 atomic percent, for the catalytic oxidation of ahydrogenontaining fuel or the catalytic reduction of oxygen.

The present invention is still further directed to a method for theelectrochemical conversion of a hydrogen-containing fuel and oxygen toreaction products and electricity in a fuel cell comprising an anode, acathode, a proton exchange membrane therebetween, a ternary catalystwhich comprises platinum, manganese, and cobalt, wherein theconcentration of manganese is no greater than 20 atomic percent, and anelectrically conductive external circuit connecting the anode andcathode. The method comprises contacting the hydrogen-containing fuel orthe oxygen and the ternary catalyst to catalytically oxidize thehydrogen-containing fuel or catalytically reduce the oxygen.

The present invention is still further directed to an unsupportedternary catalyst layer on a surface of an electrolyte membrane or anelectrode, said unsupported ternary catalyst layer comprising platinum,manganese, and cobalt.

The foregoing and other features and advantages of the present inventionwill become more apparent from the following description andaccompanying figures.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a schematic structural view showing members of a fuel cell.

FIG. 2 is a cross-sectional side view of a fuel cell.

FIG. 3 is a photograph of an electrode array comprising thin-film alloycompositions deposited on individually addressable electrodes preparedas described in Example 1.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to a ternary, metal-containingsubstance having electrocatalytic activity for use in, for example, fuelcells (e.g., an electrocatalyst). In one embodiment the ternary,metal-containing substance is an alloy of the components. However, it isto be noted that the substance (e.g., electrocatalyst) may be a mixtureof discrete amounts of the components (e.g., a mixture of metal powdersor a mixture of deposits), wherein a discrete amount of the componentsmay comprise a single component or a combination of components (e.g., analloy).

In general, it is desirable to decrease the concentration of noblemetals (especially platinum) to reduce the cost of an electrocatalyticalloy. However, as the concentrations of noble metals are decreased, theelectrocatalyst alloy may become more susceptible to corrosion, and/orthe activity may be diminished. Thus, it is desirable to achieve themost activity per weight percent of noble metals without compromising,for example, the life cycle of the fuel cell in which theelectrocatalyst is placed (see, end current density/weight fraction ofplatinum as set forth in Tables A and B, infra). Additionally, thecomposition of the present invention is preferably optimized to limitnoble metal concentration while improving corrosion resistance and/oractivity, as compared to platinum.

The present invention thus is directed to a ternary metal-containingsubstance (e.g., alloy or catalyst) that comprises platinum, manganeseand cobalt, wherein the concentration of manganese is less than 20atomic percent. Furthermore, the ternary substance of the presentinvention comprises amounts of platinum, manganese and cobalt which aresufficient for the metals, present therein, to play a role in thecatalytic activity and/or crystallographic structure of, for example,the alloy. Stated another way, the concentrations of platinum, manganeseand cobalt are such that the presence of the metals would not beconsidered an impurity. For example, the concentrations of each ofplatinum, manganese and cobalt are at least about 0.1, 0.5. 1, or even 5atomic percent.

Accordingly, in addition to comprising less than 20 atomic percentmanganese (e.g., less than about 15, about 12, about 10, about 8 or evenabout 5 atomic percent), in at least some embodiments the ternarycatalyst of the present invention may comprise less than about 70, about60 or about 50 atomic percent platinum. The ternary catalyst may alsocomprises less than about 70, about 60 or about 50 atomic percentcobalt. Furthermore, in some embodiments the concentration of manganesemay range from about 5 to less than 15 atomic percent, or from about 8to less than 12 atomic percent. In these or other embodiments, theconcentration of platinum may range from about 10 to about 70 atomicpercent, from about 20 to about 60 atomic percent, or from about 30 toabout 50 atomic percent. In these or still other embodiments, theconcentration of cobalt may range from about 10 to about 70 atomicpercent, from about 20 to about 60 atomic percent, or from about 30 toabout 50 atomic percent. Additionally, in one or more embodiments, thesum of manganese and cobalt may range from about 20 to about 60 atomicpercent, or about 30 to about 50 atomic percent.

In this regard it is to be noted that in one embodiment the substance ofthe present invention consists essentially of the foregoing metals(e.g., it most likely will contain some degree of impurities that do notplay a role in the catalytic activity and/or crystallographic structureof the catalyst). However, in other embodiments it is possible that thesubstance may comprise other constituents as intentional additions. Forexample, many metal alloys may comprise oxygen and/or carbon, either asan impurity or as a desired alloy constituent. In view of the foregoing,the alloys of the present invention, while maintaining the same relativeamounts of the constituents disclosed herein (i.e., platinum, manganese,cobalt), may comprise less than 100 percent of the disclosed atoms.Thus, in several embodiments of the present invention the totalconcentration of the disclosed atoms is greater than about 70, 80, 90,95, or 99 atomic percent of the substance (e.g., electrocatalyst alloy).

In view of the foregoing, it is to be noted that in some embodiments thepresent invention is directed to a ternary catalyst or alloy comprisingabout 10 to less than 20 atomic percent manganese, about 40 to about 60atomic percent platinum, and about 25 to about 45 atomic percent cobalt.In other embodiments, the present invention is directed to a ternarycatalyst or alloy comprising about 12 to less than 18 atomic percentmanganese, about 45 to about 55 atomic percent platinum, and about 30 toabout 40 atomic percent cobalt.

A specific Pt—Mn—Co alloy composition which has been found to exhibit agreater oxygen reduction activity than a platinum standard (i.e.,Electrode 56 of Table A, infra) is the alloy Pt₄₀Mn₂₀Co₄₀ (i.e.,Electrode Number 9 of Table A). Additional specific alloy compositionswhich have been found to exhibit a greater oxygen reduction than aplatinum standard are the alloys Pt₅₉Mn₂₆Co₁₅, Pt₄₆Mn₁₃Co₄₁,Pt₅₉Mn₁₃Co₂₈, Pt₇₂Mn₁₃Co₁₅, and Pt₄₇Mn₂₅Co₂₈ (i.e., Electrode Numbers47, 53, 54, 55, and 46 of Table B, respectively). Still more specificalloy compositions which have been found to exhibit a greater oxygenreduction than a platinum standard are the alloys Pt₄₇Mn16Co₃₇,Pt₄₇Mn16Co₃₇, Pt₅₀Mn₁₁Co₃₉, and Pt₅₁Mn₁₃Co₃₆ (i.e., Powders HFC 14, 15,126, and 129 of Table C, infra).

Formation of an Electrocatalyst Alloy

The electrocatalyst alloys of the present invention may be formed by avariety of methods. For example, the appropriate amounts of theconstituents may be mixed together and heated to a temperature above therespective melting points to form a molten solution of the metals whichis cooled and allowed to solidify. Typically, electrocatalysts are usedin a powder form to increase the surface area which increases the numberof reactive sites and leads to improved efficiency of the cell. Thus,the formed metal alloy may be transformed into a powder after beingsolidified (e.g., by grinding) or during solidification (e.g., sprayingmolten alloy and allowing the droplets to solidify). It may, however, beadvantageous to evaluate alloys for electrocatalytic activity in anon-powder form (see, Examples 1 and 2, Infra).

To further increase surface area and efficiency, an electrocatalystalloy for use in a fuel cell may be deposited over the surface ofelectrically conductive supports (e.g., carbon black). One method forloading an electrocatalyst alloy onto supports typically comprisesdepositing metal precursor compounds onto the supports, and convertingthe precursor compounds to metallic form and alloying the metals using aheat-treatment in a reducing atmosphere (e.g., an atmosphere comprisingan inert gas such as argon). One method for depositing the precursorcompounds involves chemical precipitation of precursor compounds ontothe supports. The chemical precipitation method is typicallyaccomplished by mixing supports and sources of the precursor compounds(e.g., an aqueous solution comprising one or more inorganic metallicsalts) at a concentration sufficient to obtain the desired loading ofthe electrocatalyst on the supports and then precipitation of theprecursor compounds is initiated (e.g., by adding an ammonium hydroxidesolution). The slurry is then typically filtered from the liquid undervacuum, washed with deionized water, and dried to yield a powder thatcomprises the precursor compounds on the supports.

Another method for depositing the precursor compounds comprises forminga suspension comprising a solution and supports suspended therein,wherein the solution comprises a solvent portion and a solute portionthat comprises the constituents of the precursor compound(s) beingdeposited. The suspension is frozen to deposit (e.g., precipitate) theprecursor compound(s) on the particles. The frozen suspension isfreeze-dried to remove the solvent portion and leave a freeze-driedpowder comprising the supports and the deposits of the precursorcompound(s) on the supports.

The temperature reached during the thermal treatment is typically atleast as high as the decomposition temperature(s) for the precursorcompound(s) and not be so high as to result in degradation of thesupports and agglomeration of the supports and/or the electrocatalystdeposits. Typically the temperature is between about 60° C. and about1100° C. Inorganic metal-containing compounds typically decompose attemperatures between about 600 and about 1000° C.

The duration of the heat treatment is typically at least sufficient tosubstantially convert the precursor compounds to the desired state. Ingeneral, the temperature and time are inversely related (i.e.,conversion is accomplished in a shorter period of time at highertemperatures and vice versa). At the temperatures typical for convertingthe inorganic metal-containing compounds to a metal alloy set forthabove, the duration of the heat treatment is typically at least about 30minutes. In one embodiment, the duration is between about 2 and about 8hours.

Unsupported Catalyst or Alloys in Electrode/Fuel Cell Applications

It is to be noted that, in another embodiment of the present invention,the ternary metal substance (e.g., catalyst or alloy) may beunsupported; that is, it may be employed in the absence of a supportparticle. More specifically, it is to be noted that in anotherembodiment of the present invention the ternary metal catalyst or alloymay be directly deposited (e.g., sputtered) onto, for example, (i) asurface of one or both of the electrodes (e.g., the anode, the cathodeor both), and/or (ii) one or both surfaces of a polyelectrolytemembrane, and/or (iii) some other surface, such as a backing for themembrane (e.g., carbon paper).

In this regard it is to be further noted that each component (e.g.,metal) of the ternary catalyst or alloy may be deposited separately,each for example as a separate layer on the surface of the electrode,membrane, etc. Alternatively, two or more components may be deposited atthe same time. Additionally, in the case of an alloy, the alloy may beformed and then deposited, or the components thereof may be depositedand then the alloy subsequently formed thereon.

Deposition of the component(s) may be achieved using means known in theart, including for example known sputtering technique (see, e.g., PCTApplication No. WO 99/16137). Generally speaking, however, in oneapproach sputter-deposition is achieved by creating, within a vacuumchamber in an inert atmosphere, a voltage differential between a targetcomponent material and the surface onto which the target component is tobe deposited, in order to dislodge particles from the target componentmaterial which are then attached to the surface of, for example, anelectrode or electrolyte membrane, thus forming a coating of the targetcomponent thereon. In one embodiment, the components are deposited on apolymeric electrolyte membrane, including for example (i) a copolymermembrane of tetrafluorethylene and perfluoropolyether sulfonic acid(such as the membrane material sold under the trademark NAFION), (ii) aperfluorinated sulfonic acid polymer (such as the membrane material soldunder the trademark ACIPLEX), (iii) polyethylene sulfonic acid polymers,(iv) polyketone sulfonic acids, (v) polybenzimidazole doped withphosphoric acid, (vi) sulfonated polyether sulfones, and (vii) otherpolyhydrocarbon-based sulfonic acid polymers.

It is to be noted that the specific amount of each metal or component ofthe ternary catalyst or alloy may be controlled independently, in orderto tailor the composition to a given application. In some embodiments,however, the amount of each deposited component may be less than about 5mg/cm² of surface area (e.g., electrode surface area, membrane surfacearea, etc.), less than about 1 mg/cm², less than about 0.5 mg/cm², lessthan about 0.1 mg/cm², or even less than about 0.05 mg/cm².Alternatively, in some embodiments the amount of the depositedcomponent, or alloy, may range from about 0.5 mg/cm² to less than about5 mg/cm², or from about 0.1 mg/cm² to less than about 1 mg/cm².

It is to be further noted that the specific amount of each component,and/or the conditions under which the component is deposited, may becontrolled in order to control the resulting thickness of the component,or alloy, layer on the surface of the electrode, electrolyte membrane,etc. For example, as determined by means known in the art (e.g.,scanning electron microscopy or Rutherford back scatteringspectrophotometric method), the deposited layer may have a thicknessranging from several angstroms (e.g., about 2, 4, 6, 8, 10 or more) toseveral tens of angstroms (e.g., about 20, 40, 60, 80, 100 or more), upto several hundred angstroms (e.g., about 200, 300, 400, 500 or more).Additionally, after all of the components have been deposited, and/oralloyed (or, alternatively, after the alloy has been deposited), thelayer of the multi-component metal substance of the present inventionmay have a thickness ranging from several tens of angstroms (e.g., about20, 40, 60, 80, 100 or more), up to several hundred angstroms (e.g.,about 200, 400, 600, 800, 1000, 1500 or more). Thus, in differentembodiments the thickness may be, for example, between about 10 andabout 500 angstroms, between about 20 and about 200 angstroms, orbetween about 40 and about 100 angstroms.

It is to be still further noted that in embodiments wherein a ternarycatalyst or alloy (or the components thereof) are deposited as a thinfilm on the surface of, for example, an electrode or electrolytemembrane, the composition of the deposited catalyst or alloy may be aspreviously described herein. Alternatively, however, the ternarycatalyst or alloy may comprise more than 20 atomic percent manganesewhen employed in the absence of a support particle (e.g., a carbonsupport particle). For example, in such an embodiment the concentrationof manganese may be about 60 atomic percent, about 50 atomic percent,about 40 atomic percent, or even about 30 atomic percent. Further, theconcentration of platinum may range from about 20 to about 60 atomicpercent, or about 30 to about 50 atomic percent. In these or otherembodiments, the concentration of cobalt may range from about 10 toabout 60 atomic percent, or from about 20 to about 40 atomic percent.

Incorporation of the Electrocatalyst Compositions in a Fuel Cell

Although the alloy compositions of the present invention can be used inany type of fuel cell (e.g., phosphoric acid, molten carbonate, solidoxide, potassium hydroxide, and proton exchange membrane), they areparticularly suited for use in proton exchange membrane fuel cells. Asshown in FIGS. 1 and 2, a fuel cell, generally indicated 21, comprises afuel electrode (anode) 2 and an air electrode, oxidizer electrode(cathode) 3. In between the electrodes 2 and 3, a proton exchangemembrane 1 serves as an electrolyte and it is usually a strongly acidicion exchange membrane such as a perfluorosulphonic acid-based membrane.Preferably, the proton exchange membrane 1, the anode 2, and the cathode3 are integrated into one body to minimize contact resistance betweenthe electrodes and the proton exchange membrane. Current collectors 4and 5 engage the anode and the cathode, respectively. A fuel chamber 8and an air chamber 9 contain the respective reactants and are sealed bysealants 6 and 7, respectively.

In general, electricity is generated by hydrogenontaining fuelcombustion (i.e., the hydrogen-containing fuel and oxygen react to formwater, carbon dioxide and electricity). This is accomplished in theabove-described fuel cell by introducing the hydrogenontaining fuel Finto the fuel chamber 8, while oxygen O (preferably air) is introducedinto the air chamber 9, whereby an electric current can be immediatelytransferred between the current collectors 4 and 5 through an outercircuit (not shown). Ideally, the hydrogen-containing fuel is oxidizedat the anode 2 to produce hydrogen ions, electrons, and possibly carbondioxide gas. The hydrogen ions migrate through the strongly acidicproton exchange membrane 1 and react with oxygen and electronstransferred through the outer circuit to the cathode 3 to form water. Ifthe hydrogen-containing fuel F is methanol, it is preferably introducedas a dilute acidic solution to enhance the chemical reaction, therebyincreasing power output (e.g., a 0.1 M methanol/0.5 M sulfuric acidsolution).

To prevent the loss of ionic conduction in the proton exchangemembranes, they typically remain hydrated during operation of the fuelcell. As a result, the material of the proton exchange membrane istypically selected to be resistant to dehydration at temperatures up tobetween about 100 and about 120° C. Proton exchange membranes usuallyhave reduction and oxidation stability, resistance to acid andhydrolysis, sufficiently low electrical resistivity (e.g., <10 Ω·cm),and low hydrogen or oxygen permeation. Additionally, proton exchangemembranes are usually hydrophilic. This ensures proton conduction (byreversed diffusion of water to the anode), and prevents the membranefrom drying out thereby reducing the electrical conductivity. For thesake of convenience, the layer thickness of the membranes is typicallybetween 50 and 200 μm. In general, the foregoing properties are achievedwith materials which have no aliphatic hydrogen-carbon bonds, which, forexample, are achieved by replacing hydrogen with fluorine or by thepresence of aromatic structures; the proton conduction results from theincorporation of sulfonic acid groups (high acid strength). Suitableproton-conducting membranes also include perfluorinated sulfonatedpolymers such as NAFION and its derivatives produced by E.I. du Pont deNemours & Co., Wilmington, Del. NAFION is based on a copolymer made fromtetrafluoroethylene and perfluorovinylether, and is provided withsulfonic groups working as ion-exchanging groups. Other suitable protonexchange membranes are produced with monomers such as perfluorinatedcompounds (e.g., octafluorocyclobutane and pertluorobenzene), or evenmonomers with C—H bonds which, in a plasma polymer, do not form anyaliphatic H atoms which could constitute attack sites for oxidativebreakdown.

The electrodes of the present invention comprise the electrocatalystcompositions of the present invention and an electrode substrate uponwhich the electrocatalyst is deposited. In one embodiment theelectrocatalyst alloy is directly deposited on the electrode substrate.In another embodiment the electrocatalyst alloy is supported onelectrically conductive supports and the supported electrocatalyst isdeposited on the electrode substrate. The electrode may also comprise aproton conductive material that is in contact with the electrocatalyst.The proton conductive material may facilitate contact between theelectrolyte and the electrocatalyst, and may thus enhance fuel cellperformance. Preferably, the electrode is designed to increase cellefficiency by enhancing contact between the reactant (i.e., fuel oroxygen), the electrolyte and the electrocatalyst. In particular, porousor gas diffusion electrodes are typically used since they allow thefuel/oxidizer to enter the electrode from the face of the electrodeexposed to the reactant gas stream (back face), and the electrolyte topenetrate through the face of the electrode exposed to the electrolyte(front face), and reaction products, particularly water, to diffuse outof the electrode.

The electrically conductive support particles typically comprise aninorganic material such as carbon. However, the support particles maycomprise an organic material such as an electrically conductive polymer(see, U.S. Pat. Appln. 2002/0132040 A1). Carbon supports may bepredominantly amorphous or graphitic and they may be preparedcommercially, or specifically treated to increase their graphitic nature(e.g., heat treated at a high temperature in vacuum or in an inert gasatmosphere) thereby increasing corrosion resistance. For example, it maybe oil furnace black, acetylene black, graphite paper, carbon fabric, orcarbon aerogel. A carbon aerogel preferably has an electricalconductivity of between 10⁻² and 10³ Ω⁻¹·cm⁻¹ and a density of between0.06 and 0.7 g/cm³; the pore size is between 20 and 100 nm (porosity upto about 95%). Carbon black support particles may have a Brunauer,Emmett and Teller (BET) surface area up to about 2000 m²/g. It has beenreported that satisfactory results are achieved using carbon blacksupport particles having a high mesoporous area, e.g., greater thanabout 75 m²/g (see, Catalysis for Low Temperature Fuel Cells Part 1: TheCathode Challenges, T. R. Ralph and M. P. Hogarth, Platinum Metals Rev.,2002, 46, (1), p. 3-14). Experimental results to date indicate that asurface area of about 500 m²/g is preferred.

Preferably, the proton exchange membrane, electrodes, andelectrocatalyst materials are in contact with each other. This istypically accomplished by depositing the electrocatalyst either on theelectrode, or on the proton exchange membrane, and then placing theelectrode and membrane in contact. The alloy electrocatalysts of thisinvention can be deposited on either the electrode or the membrane by avariety of methods, including plasma deposition, powder application (thepowder may also be in the form of a slurry, a paste, or an ink),chemical plating, and sputtering. Plasma deposition generally entailsdepositing a thin layer (e.g., between 3 and 50 μm, preferably between 5and 20 μm) of an electrocatalyst composition on the membrane usinglow-pressure plasma. By way of example, an organic platinum compoundsuch as trimethylcyclopentadienylplatinum is gaseous between 10⁻⁴ and 10mbar and can be excited using radio-frequency, microwaves, or anelectron cyclotron resonance transmitter to deposit platinum on themembrane. According to another procedure, electrocatalyst powder isdistributed onto the proton exchange membrane surface and integrated atan elevated temperature under pressure. If, however, the amount ofelectrocatalyst particles exceeds about 2 mg/cm² the inclusion of abinder such as polytetrafluoroethylene is common. Further, theelectrocatalyst may be plated onto dispersed small support particles(e.g., the size is typically between 20 and 200 Å, and more preferablybetween about 20 and 100 Å). This increases the electrocatalyst surfacearea which in turn increases the number of reaction sites leading toimproved cell efficiency. In one such chemical plating process, forexample, a powdery carrier material such as conductive carbon black iscontacted with an aqueous solution or aqueous suspension (slurry) ofcompounds of metallic components constituting the alloy to permitadsorption or impregnation of the metallic compounds or their ions on orin the carrier. Then, while the slurry is stirred at high speed, adilute solution of suitable fixing agent such as ammonia, hydrazine,formic acid, or formalin is slowly added dropwise to disperse anddeposit the metallic components on the carrier as insoluble compounds orpartly reduced fine metal particles.

The loading, or surface concentration, of an electrocatalyst on themembrane or electrode is based in part on the desired power output andcost for a particular fuel cell. In general, power output increases withincreasing concentration; however, there is a level beyond whichperformance is not improved. Likewise, the cost of a fuel cell increaseswith increasing concentration. Thus, the surface concentration ofelectrocatalyst is selected to meet the application requirements. Forexample, a fuel cell designed to meet the requirements of a demandingapplication such as an extraterrestrial vehicle will usually have asurface concentration of electrocatalyst sufficient to maximize the fuelcell power output. For less demanding applications, economicconsiderations dictate that the desired power output be attained with aslittle electrocatalyst as possible. Typically, the loading ofelectrocatalyst is between about 0.01 and about 6 mg/cm². Experimentalresults to date indicate that in some embodiments the electrocatalystloading is preferably less than about 1 mg/cm², and more preferablybetween about 0.1 and 1 mg/cm².

To promote contact between the collector, electrode, electrocatalyst,and membrane, the layers are usually compressed at high temperature. Thehousings of the individual fuel cells are configured in such a way thata good gas supply is ensured, and at the same time the product water canbe discharged properly. Typically, several fuel cells are joined to formstacks, so that the total power output is increased to economicallyfeasible levels.

In general, the electrocatalyst compositions and fuel cell electrodes ofthe present invention may be used to electrocatalyze any fuel containinghydrogen (e.g., hydrogen and reformated-hydrogen fuels). Also,hydrocarbon-based fuels may be used including saturated hydrocarbonssuch as methane (natural gas), ethane, propane and butane; garbageoff-gas; oxygenated hydrocarbons such as methanol and ethanol; andfossil fuels such as gasoline and kerosene; and mixtures thereof.

To achieve the full ion-conducting property of proton exchangemembranes, in some embodiments suitable acids (gases or liquids) aretypically added to the fuel. For example, SO₂, SO₃, sulfuric acid,trifluoromethanesulfonic acid or the fluoride thereof, also stronglyacidic carboxylic acids such as trifluoroacetic acid, and volatilephosphoric acid compounds may be used (“Ber. Bunsenges. Phys. Chem.”,Volume 98 (1994), pages 631 to 635).

Fuel Cell Uses

As set forth above, the alloy compositions of the present invention areuseful as electrocatalysts in fuel cells which generate electricalenergy to perform useful work. For example, the alloy compositions maybe used in fuel cells which are in electrical utility power generationfacilities; uninterrupted power supply devices; extraterrestrialvehicles; transportation equipment such as heavy trucks, automobiles,and motorcycles (see, Fuji et al., U.S. Pat. No. 6,048,633; Shinkai etal., U.S. Pat. No. 6,187,468; Fuji et al., U.S. Pat. No. 6,225,011; andTanaka et al., U.S. Pat. No. 6,294,280); residential power generationsystems; mobile communications equipment such as wireless telephones,pagers, and satellite phones (see, Prat et al., U.S. Pat. No. 6,127,058and Kelley et al., U.S. Pat. No. 6,268,077); mobile electronic devicessuch as laptop computers, personal data assistants, audio recordingand/or playback devices, digital cameras, digital video cameras, andelectronic game playing devices; military and aerospace equipment suchas global positioning satellite devices; and robots.

EXAMPLE 1 Forming Electrocatalytic Alloys on Individually AddressableElectrodes

The electrocatalyst alloy compositions set forth in Tables A and B,infra, were prepared using the combinatorial techniques disclosed inWarren et al., U.S. Pat. No. 6,187,164; Wu et al., U.S. Pat. No.6,045,671; Strasser, P., Gorer, S. and Devenney, M., CombinatorialElectrochemical Techniques For The Discovery of New Fuel-Cell CathodeMaterials, Nayayanan, S. R., Gottesfeld, S. and Zawodzinski, T., eds.,Direct Methanol Fuel Cells, Proceedings of the Electrochemical Society,New Jersey, 2001, p. 191; and Strasser, P., Gorer, S. and Devenney, M.,Combinatorial Electrochemical Strategies For The Discovery of NewFuel-Cell Electrode Materials, Proceedings of the InternationalSymposium on Fuel Cells for Vehicles, 41st Battery Symposium, TheElectrochemical Society of Japan, Nagoya 2000, p. 153. For example, anarray of independent electrodes (with areas of between about 1 and 3mm²) may be fabricated on inert substrates (e.g., glass, quartz,sapphire alumina, plastics, and thermally treated silicon). Theindividual electrodes were located substantially in the center of thesubstrate, and were connected to contact pads around the periphery ofthe substrate with wires. The electrodes, associated wires, and contactpads were fabricated from a conducting material (e.g., titanium, gold,silver, platinum, copper or other commonly used electrode materials).

Specifically, the alloy compositions set forth in Tables A and B wereprepared using a photolithography/RF magnetron sputtering technique (GHzrange) to deposit thin-film alloys on arrays of 64 individuallyaddressable electrodes. A quartz insulating substrate was provided andphotolithographic techniques were used to design and fabricate theelectrode patterns on it. By applying a predeternined amount ofphotoresist to the substrate, photolyzing preselected regions of thephotoresist, removing those regions that have been photolyzed (e.g., byusing an appropriate developer), depositing a layer of titanium about500 nm thick using RF magnetron sputtering over the entire surface andremoving predetermined regions of the deposited titanium (e.g. bydissolving the underlying photoresist), intricate patterns ofindividually addressable electrodes were fabricated on the substrate.

Referring to FIG. 3, the fabricated array 20 consisted of 64individually addressable electrodes 21 (about 1.7 mm in diameter)arranged in an 8×8 square that were insulated from each other (byadequate spacing) and from the substrate 24 (fabricated on an insulatingsubstrate), and whose interconnects 22 and contact pads 23 wereinsulated from the electrochemical testing solution (by the hardenedphotoresist or other suitable insulating material).

After the initial array fabrication and prior to depositing theelectrocatalyst alloys for screening, a patterned insulating layercovering the wires and an inner portion of the peripheral contact pads,but leaving the electrodes and the outer portion of the peripheralcontact pads exposed (preferably approximately half of the contact padis covered with this insulating layer) was deposited. Because of theinsulating layer, it is possible to connect a lead (e.g., a pogo pin oran alligator clip) to the outer portion of a given contact pad andaddress its associated electrode while the array is immersed insolution, without having to worry about reactions that can occur on thewires or peripheral contact pads. The insulating layer was a hardenedphotoresist, but any other suitable material known to be insulating innature could have been used (e.g., glass silica, alumina, magnesiumoxide, silicon nitride, boron nitride, yttrium oxide, or titaniumdioxide).

Following the creation of the titanium coated array, a steel mask having64 holes (1.7 mm in diameter) was pressed onto the substrate to preventdeposition of sputtered material onto the insulating resist layer. Thedeposition of the electrocatalyst alloys was also accomplished using RFmagnetron sputtering and a two shutter masking system as described by Wuet al. which enable the deposition of material onto 1 or more electrodesat a time. Each individual thin-film electrocatalyst alloy is created bya super lattice deposition method. For example, when preparing a ternaryalloy electrocatalyst composition, metals M1, M2 and M3 are to bedeposited and alloyed onto one electrode. First, a metal M1 sputtertarget is selected and a thin film of M1 having a defined thickness isdeposited on the electrode. This initial thickness is typically fromabout 3 to about 12 Å. After this, metal M2 is selected as the sputtertarget and a layer of M2 is deposited onto the layer of M1. Thethickness of M2 layer is also from about 3 to about 12 Å. Thethicknesses of the deposited layers are in the range of the diffusionlength of the metal atoms (e.g., about 10 to about 30 Å) which allowsin-situ alloying of the metals. Then, a layer of M3 is deposited ontothe M1-M2 alloy forming a M1-M2-M3 alloy film. As a result of the threedeposition steps, an alloy thin-film (9-25 Å thickness) of the desiredstoichiometry is created. This concludes one deposition cycle. In orderto achieve the desired total thickness of a cathode electrocatalystsmaterial, deposition cycles are repeated as necessary which results inthe creation of a super lattice structure of a defined total thickness(typically about 700 Å). Although the number, thickness (stoichiometry)and order of application of the individual metal layers may bedetermined manually, it is desirable to utilize a computer program todesign an output file which contains the information necessary tocontrol the operation of the sputtering device during the preparation ofa particular library wafer (i.e., array). One such computer program isthe LIBRARY STUDIO software available from Symyx Technologies, Inc. ofSanta Clara, Calif. and described in European Patent No. 1080435 B1. Thecompositions of several as sputtered alloy compositions were analyzedusing x-ray fluorescence (XRF) to confirm that they were consistent withdesired compositions (chemical compositions determined using x-rayfluorescence are within about 5% of the actual composition).

Arrays were prepared to evaluate the specific alloy compositions setforth in Tables A and B below. On each array one electrode consistedessentially of platinum and it served as an internal standard for thescreening operation. Additionally, the results for the alloys may beevaluated against an external platinum standard comprising an array of64 platinum electrodes in which the oxygen reduction activity of the 64platinum electrodes averaged −0.35 mA/cm² at +0.1V vs. a mercury/mercurysulfate electrode to determine the experimental error of the oxygenreduction test. TABLE A End Current End Current Density Density/(Absolute Weight Electrode activity) Fraction of Co Mn Pt Number mA/cm2Pt atomic % atomic % atomic % 9 −1.766 −2.557 40.25 20.04 39.71 56−0.781 −0.781 0.00 0.00 100.00 57 −0.551 −0.792 20.14 40.11 39.75 58−0.179 −0.390 40.17 40.00 19.82 10 −0.172 −0.378 60.21 19.99 19.80 15−0.036 −0.077 20.10 60.06 19.84 16 0.003 0.004 20.15 20.07 59.78

TABLE B End End Current Current Relative Density Density/ Activity(Absolute Weight Compared Pt Mn Co Electrode activity) Fraction toatomic atomic atomic Number mA/cm² of Pt Internal Pt % % % 47 −0.917−1.098 1.882 59.26 26.35 14.38 53 −0.847 −1.138 1.740 46.43 12.53 41.0454 −0.830 −1.004 1.705 58.56 13.02 28.43 55 −0.678 −0.757 1.393 71.6713.54 14.79 46 −0.516 −0.687 1.061 46.97 25.35 27.68 64 −0.487 −0.4871.000 100.00 0.00 0.00 39 −0.190 −0.251 0.391 47.52 38.48 14.00 2 −0.125−0.494 0.257 9.01 47.01 43.99 45 −0.124 −0.190 0.255 35.57 24.43 40.0037 −0.112 −0.209 0.231 25.25 35.74 39.02 1 −0.106 −0.698 0.218 4.9745.43 49.60 9 −0.102 −0.404 0.209 8.95 40.04 51.01 38 −0.093 −0.1410.191 35.97 37.06 26.97 52 −0.076 −0.118 0.156 35.17 12.08 52.75 12−0.063 −0.125 0.129 22.82 44.67 32.51 3 −0.062 −0.180 0.128 13.33 48.7037.98 10 −0.054 −0.157 0.111 13.24 41.48 45.28 17 −0.050 −0.147 0.10213.16 34.35 52.50 43 −0.039 −0.104 0.080 15.10 22.77 62.14 44 −0.036−0.068 0.074 24.97 23.57 51.46 21 −0.035 −0.055 0.072 33.74 40.03 26.2242 −0.034 −0.054 0.069 33.02 15.67 51.32 49 −0.032 −0.052 0.067 32.787.78 59.44 13 −0.028 −0.048 0.057 28.18 46.46 25.36 20 −0.026 −0.0450.053 27.98 38.44 33.57 35 −0.021 −0.033 0.043 33.26 23.67 43.07 34−0.019 −0.034 0.039 27.59 22.74 49.66 14 −0.018 −0.028 0.037 33.99 48.3917.61 7 −0.017 −0.026 0.035 34.25 56.88 8.87 41 −0.016 −0.028 0.03227.40 15.06 57.54 31 −0.014 −0.021 0.029 36.38 49.98 13.64 11 −0.014−0.033 0.029 17.85 43.01 39.13 4 −0.014 −0.032 0.028 17.97 50.51 31.5133 −0.013 −0.026 0.027 22.36 21.89 55.75 25 −0.013 −0.030 0.026 17.6228.30 54.08 6 −0.009 −0.015 0.018 28.38 54.59 17.03 51 −0.008 −0.0150.016 24.71 11.66 63.64 28 −0.006 −0.010 0.013 33.50 31.79 34.71 30−0.005 −0.008 0.009 25.52 48.18 26.30 29 −0.003 −0.009 0.007 15.42 46.5038.08 27 −0.003 −0.005 0.006 27.79 30.54 41.67 5 −0.002 −0.004 0.00422.98 52.47 24.55 18 0.000 0.001 −0.001 17.74 35.61 46.65 26 0.002 0.004−0.004 22.51 29.38 48.11 36 0.002 0.005 −0.004 15.26 34.51 50.23 500.002 0.005 −0.004 14.94 11.27 73.79

EXAMPLE 2 Screening Alloys for Electrocatalytic Activity

The alloy compositions set forth in Tables A and B (set forth above)that were synthesized on arrays according to the method set forth inExample 1 were screened according to the protocol set forth below forelectrochemical reduction of molecular oxygen to water to determinerelative electrocatalytic activity against the internal and/or externalplatinum standard.

In general, the array wafers were assembled into an electrochemicalscreening cell and a screening device established an electrical contactbetween the 64 electrode electrocatalysts (working electrodes) and a64-channel multi channel potentiostat used for the screening.Specifically, each wafer array was placed into a screening device suchthat all 64 spots are facing upward and a tube cell body that wasgenerally annular and having an inner diameter of about 2 inches (5 cm)was pressed onto the upward facing wafer surface. The diameter of thistubular cell was such that the portion of the wafer with the squareelectrode array formed the base of a cylindrical volume while thecontact pads were outside the cylindrical volume. A liquid ionicsolution (electrolyte) was poured into this cylindrical volume and acommon counter electrode (i.e., platinum gauze), as well as a commonreference electrode (e.g., mercury/mercury sulfate reference electrode(MMS)), were placed into the electrolyte solution to close theelectrical circuit.

The protocol comprised placing a rotator shaft with blades into theelectrolyte to provide forced convection-diffusion conditions during thescreening. The rotation rate was typically between about 300 to about400 rpm. Depending on the screening experiment either argon or pureoxygen was bubbled through the electrolyte during the measurements.Argon served to remove O₂ gas in the electrolyte to simulate O₂-freeconditions used for the initial conditioning of the electrocatalysts.The introduction of pure oxygen served to saturate the electrolyte withoxygen for the oxygen reduction reaction. During the screening, theelectrolyte was maintained at 60° C. and the rotation rate was constant.Three groups of tests were performed to screen the activity of theelectrocatalysts. The electrolyte (1 M HClO₄) was purged with argon forabout 20 minutes prior to the electrochemical measurements. The firstgroup of tests comprised cyclic voltammetric measurements while purgingthe electrolyte with argon. Specifically, the first group of testscomprised:

-   -   a. a potential sweep from about OCP to about +0.3 V to about        −0.63 V and back to about +0.3 V at a rate of about 20 mV/s;    -   b. twelve consecutive potential sweeps from OCP to about +0.3 V        to about −0.7 V and back to about +0.3 V at a rate of about 200        mV/s; and    -   c. a potential sweep from about OCP to about +0.3 V to about        −0.63 V and back to about +0.3 V at a rate of about 20 mV/s.        The electrolyte was then purged with oxygen for approximately 30        minutes. The following second group of tests were performed        while continuing to purge with oxygen:    -   a. measuring the open circuit potential (OCP) for a minute;        then, starting at OCP the voltage was swept down to about −0.4 V        at a rate of about 10 mV/s;    -   b. measuring the OCP for a minute; then applying a potential        step from OCP to about +0.1 V while measuring the current for        about 5 minutes; and    -   c. measuring the OCP for a minute; then applying a potential        step from OCP to about +0.2 V while monitoring the current for        about 5 minutes.        The third group of tests comprised a repeat of the second group        of tests after about one hour from completion of the second        group of tests. The electrolyte was continually stirred and        purged with oxygen during the waiting period. All the foregoing        test voltages are with reference to a mercury/mercury sulfate        (MMS) electrode.

The specific alloy compositions set forth in Tables A and B wereprepared and screened in accordance with the above-described methods andthe results are set forth therein. The screening results in Tables A andB are for the third test group steady state currents at +0.1 V MMS. Thecurrent value reported (End Current Density) is the result of averagingthe last three current values of the chronoamperometric test normalizedfor geometric surface area.

EXAMPLE 3 Synthesis of Supported Electrocatalyst Alloys

The synthesis of Pt46Mn₁₃Co₄₁, Pt₃₀Mn₃₀Co₄₀, and Pt46Mn₁₃Co₄₁, (see,Table C, Target Catalyst Comp.) on carbon support particles wasattempted according to different process conditions in order to evaluatethe performance of the alloys while in a state that is typically used infuel cell. To do so, the alloy component precursors were deposited orprecipitated on supported platinum powder (i.e., platinum nanoparticlessupported on carbon black particles). Platinum supported on carbon blackis commercially available from companies such as Johnson Matthey, Inc.,of New Jersey and E-Tek Div. of De-Nora, N.A., Inc., of Sommerset, N.J.Such supported platinum powder is available with a wide range ofplatinum loading. The supported platinum powder used in this example hada nominal platinum loading of about 40 percent by weight, a platinumsurface area of between about 150 and about 170 m²/g (determined by COadsorption), a combined carbon and platinum surface area between about350 and about 400 m²/g (determined by N₂ adsorption), and an averageparticle size of less than about 0.5 mm (determined by sizing screen).

Referring to Table C, the supported electrocatalyst alloys correspondingto Pt46Mn₁₃Co₄₁ (HFC 14 and 15) and Pt₃₀Mn₃₀Co₄₀ (HFC 61 and 62) weresynthesized on carbon support particles using a chemical precipitationmethod according to the following steps. First, about 0.5 g of carbonsupported platinum powder (36.4 wt % Pt) was dispersed in about 200 mLof room temperature 18 MΩ deionized water using an ultrasonic blendingdevice (e.g., an AQUASONIC 50 D set at power level 9) for about 2 hoursto form a slurry. The slurry was stirred using a magnetic stirringdevice, and while being stirred, appropriate volumes based on thetargeted electrocatalyst composition of one or more appropriatesolutions comprising the metals to be alloyed with the platinumnanoparticles were added drop-wise to the slurry (i.e., a 1 MMn(NO₃)₂.4H₂O and 1 M Co(NO₃)₂.6H₂O aqueous solutions). Specifically,for HFC14-15 0.85 ml of 1M Co(NO₃)₂.6H₂O solution and 0.27 ml of 1 M ofMn(NO₃)₂.4H₂O solution, and for HFC61-62 1.26 ml of 1 M Co(NO₃)₂.6H₂Osolution and 0.95 ml of 1 M of Mn(NO₃)₂.4H₂O solution. The stirring wascontinued and the slurry containing the dissolved metal salts was heatedto a temperature between about 60 and about 90° C. for about 1 hour.Precipitation of compounds comprising the metals was then initiated. Inthis example, a 10 wt % ammonium hydroxide aqueous solution was slowlyadded to the slurry until the slurry had a pH of about 10. The slurrywas stirred for about 15 more minutes. The slurry was then filtered fromthe liquid under vacuum after which the filtrate was washed with about150 mL of deionized water. The powder was then dried at a temperaturebetween about 60 and about 100° C. for about 4 hours to about 8 hours.

Referring to Table C, the electrocatalyst alloys corresponding toP₄₆Mn₁₃Co₄, (HFC 126-129) were formed on carbon support particles usinga freeze-drying precipitation method. The freeze-drying method comprisedforming a precursor solution comprising the desired metal atoms in thedesired concentrations. For example, to prepare the target Pt₄₆Mn₁₃Co₄,alloy compositions on supported platinum powder which had a finalnominal platinum loading of about 32.8 percent by weight (HFC 126 and127), about 0.027 g of Mn(NO3)₂.4H₂O was dissolved in about 5 ml H₂O.Next, about 0.099 g of Co(NO₃)₂.6H₂O was dissolved in the previoussolution. To prepare the target Pt₄,Mn₁₃Co₄₁ alloy compositions onsupported platinum powder which had a final nominal platinum loading ofabout 17.8 percent by weight (HFC 128 and 129), about 0.015 g ofMn(NO₃)₂.4H₂O was dissolved in about 5 ml H₂O. Next, about 0.053 g ofCo(NO₃)₂.6H₂O was added to the previous solution.

Referring to Table C, the HFC 126 and 127 solutions were introduced intoseparate HDPE (High Density Poly Ethylene) vials containing about 0.200g of supported platinum powder which had a final nominal platinumloading of about 32.8 percent by weight resulting in a black suspension.The HFC 128 and 129 solutions were introduced into separate HDPE (HighDensity Poly Ethylene) vials containing about 0.200g of supportedplatinum powder which had a final nominal platinum loading of about 17.8percent by weight resulting in a black suspension. The suspensions werehomogenized by immersing a probe of a BRANSON SONIFIER 150 into the vialand sonicating the mixture for about 1 minute at a power level of 3.

The vials containing the homogenized suspensions were then immersed in aliquid nitrogen bath for about 3 minutes to solidify the suspensions.The solid suspensions were then freezedried for about 24 hours using aLABONCO FREEZE DRY SYSTEM (Model 79480) to remove the solvent. The trayand the collection coil of the freeze dryer were maintained at about 26°C. and about −48° C, respectively, while evacuating the system (thepressure was maintained at about 0.15 mbar). After freeze-drying, eachvial contained a powder comprising the supported platinum powder, andmanganese, and cobalt precursors deposited thereon.

The recovered precursor powders (both precipitated and freeze-dried)were then subjected to a heat treatment to reduce the precursors totheir metallic state, and to alloy the metals with each other and theplatinum on the carbon black particles. One particular heat treatmentcomprised heating the powder in a quartz flow furnace with an atmospherecomprising about 6% H₂ and 94% Ar using a temperature profile of roomtemperature to about 40° C. at a rate of about 5° C./min; holding atabout 40° C. for 2 hours; increasing the temperature to about 200° C. ata rate of 5° C./min; holding at about 200° C. for two hours; increasingthe temperature at a rate of about 5° C./min to about 700 or 900° C.;holding at a max temperature of about 700 or 900° C. for a duration ofabout two or seven hours (indicated in Table C); and cooling down toroom temperature.

In order to determine the actual composition of the supportedelectrocatalyst alloys, the differently prepared alloys (e.g., bycomposition variation or by heat treatment variation) were subjected toICP elemental analysis or subjected to EDS (Electron DispersiveSpectroscopy) elemental analysis. For the latter technique, the samplepowders were compressed into 6 mm diameter pellets with a thickness ofabout 1 mm. The target alloy composition and actual composition for theprepared supported electrocatalyst alloys are also set forth in Table C.TABLE C Log Pt Catalyst Max Mass Pt Mass Mass Alloying Target MeasuredActivity Activity at Relative Activity at Target Temp for Actual Pt Ptat +0.15 V performance 0.15 V Powder Catalyst a duration CatalystLoading Loading +0.15 V MMS at +0.15 V MMS Name Comp. (° C./hrs) Comp.(wt %) (wt %) MMS (mA/mg Pt) MMS (mA/mg) HFC 10 Pt — Pt 37.9 37.9 2.11128.82 1.00 48.82 HFC 14 Pt₄₆Mn₁₃Co₄₁ 900/2 Pt₄₇Mn₁₆Co₃₇ 32.8 34.0 2.29195.67 1.52 66.53 by ICP HFC 15 Pt₄₆Mn₁₃Co₄₁ 700/2 Pt₄₇Mn₁₆Co₃₇ 32.828.4 2.45 281.34 2.18 79.90 by EDS HFC 62 Pt₃₀Mn₃₀Co₄₀ 900/2Pt₃₀Mn₂₈Co₄₂ 30.0 33.0 2.10 125.49 0.97 41.41 by ICP HFC 126Pt₄₆Mn₁₃Co₄₁ 700/2 Pt₅₀Mn₁₁Co₃₉ 32.8 31.5 2.53 340.07 2.64 107.12 by EDSHFC 127 Pt₄₆Mn₁₃Co₄₁ 700/7 not 32.8 — — — — 87.32 measured HFC 128Pt₄₆Mn₁₃Co₄₁ 700/2 not 17.8 — — — — 47.73 measured HFC 129 Pt₄₆Mn₁₃Co₄₁700/7 Pt₅₁Mn₁₃Co₃₆ 17.8 17.5 2.46 287.79 2.23 50.36 by EDS

EXAMPLE 4 Evaluating the Electrocatalytic Activity of SupportedElectrocatalysts

The supported alloy electrocatalysts set forth in Table C and formedaccording to Example 3 were subjected to electrochemical measurements toevaluate their activities. For the evaluation, the supported alloyelectrocatalysts were applied to a rotating disk electrode (RDE) as iscommonly used in the art (see, Rotating disk electrode measurements onthe CO tolerance of a high-surface area Pt/Vulcan carbon fuel cellelectrocatalyst, Schmidt et al., Journal of the Electrochemical Society(1999), 146(4), 1296-1304; and Characterization of high-surface-areaelectrocatalysts using a rotating disk electrode configuration, Schmidtet al., Journal of the Electrochemical Society (1998), 145(7),2354-2358). Rotating disk electrodes are a relatively fast and simplescreening tool for evaluating supported electrocatalysts with respect totheir intrinsic electrolytic activity for oxygen reduction (e.g., thecathodic reaction of a fuel cell).

The rotating disk electrode was prepared by depositing an aqueous-basedink that comprises the support electrocatalyst and a NAFION solution ona glassy carbon disk. The concentration of electrocatalyst powder in theNAFION solution was about 1 mg/mL. The NAFION solution comprised theperfluorinated ion-exchange resin, lower aliphatic alcohols and water,wherein the concentration of resin is about 5 percent by weight. TheNAFION solution is commercially available from the ALDRICH catalog asproduct number 27,4704. The glassy carbon electrodes were 5 mm indiameter and were polished to a mirror finish. Glassy carbon electrodesare commercially available, for example, from Pine Instrument Company ofGrove City, Pa. An aliquot of 10 μL electrocatalyst suspension was addedto the carbon substrate and allowed to dry at a temperature betweenabout 60 and 70° C. The resulting layer of NAFION and electrocatalystwas less than about 0.2 μm thick. This method produced slightlydifferent platinum loadings for each electrode made with a particularsuspension, but the variation was determined to be less than about 10percent by weight.

After being dried, the rotating disk electrode was immersed into anelectrochemical cell comprising an aqueous 0.5 M H₂SO₄ electrolytesolution maintained at room temperature. Before performing anymeasurements, the electrochemical cell was purged of oxygen by bubblingargon through the electrolyte for about 20 minutes. All measurementswere taken while rotating the electrode at about 2000 rpm, and themeasured current densities were normalized either to the glassy carbonsubstrate area or to the platinum loading on the electrode. Two groupsof tests were performed to screen the activity of the supportedelectrocatalysts. The first group of tests comprised cyclic voltammetricmeasurements while purging the electrolyte with argon. Specifically, thefirst group comprised:

-   -   a. two consecutive potential sweeps starting from OCP to about        +0.35V then to about −0.65V and back to OCP at a rate of about        50 mV/s;    -   b. two hundred consecutive potential sweeps starting from OCP to        about +0.35V then to about −0.65V and back to OCP at a rate of        about 200 mV/s; and    -   c. two consecutive potential sweeps starting from OCP to about        +0.35V then to about 40.65V and back to OCP at a rate of about        50 mV/s.        The second test comprised purging with oxygen for about 15        minutes followed by a potential sweep test for oxygen reduction        while continuing to purge the electrolyte with oxygen.        Specifically, potential sweeps from about −0.45 V to +0.35 V        were performed at a rate of about 5 mV/s to evaluate the initial        activity of the electrocatalyst as a function of potential and        to create a geometric current density plot. The electrocatalysts        were evaluated by comparing the diffusion corrected activity at        0.15 V. All the foregoing test voltages are with reference to a        mercury/mercury sulfate electrode. Also, it is to be noted that        the oxygen reduction measurements for a glassy carbon RDE        without an electrocatalyst did not show any appreciable activity        within the potential window.

The above-described supported electrocatalyst alloy compositions wereevaluated in accordance with the above-described method and the resultsare set forth in Table C. The alloy compositions Pt₄₇Mn₁₆Co₃₇,Pt₄₇Mn₁₆Co₃₇, Pt₅₀Mn₁₁Co₃₉, and Pt₅₀ Mn₁₁Co₃₆, exhibited oxygenreduction activities greater than that of carbon supported platinum. Thealloy composition Pt₃₀Mn₂₈Co₄₂ exhibited an oxygen reduction similar tocarbon supported platinum; the Pt mass activity (i.e., activity per massof platinum), however, is significantly greater than the carbonsupported platinum.

The results of the evaluation also indicate, among other things, that itmay take numerous iterations to develop a set of parameters forproducing the target alloy composition. Also evidenced by the data, isthat activity can be adjusted by changes in the processing conditions.For example, despite depositing the same amounts of metallic precursors,the HFC 15 electrocatalyst had a higher activity than the HFC 14electrocatalyst. This difference in activity may be due to severalfactors such as alloy homogeneity (e.g., an alloy, as defined below, mayhave regions in which the constituent toms show a presence or lack oforder, i.e., regions of solid solution within an ordered lattice, orsome such superstructure), changes in the lattice parameter due tochanges in the average size of component atoms, changes in particlesize, and changes in crystallographic structure/symmetry. Theramifications of structure and symmetry changes are often difficult topredict. Pt has a face-centered cubic (fcc) structure whereas thestructures of Mn and Co are more complex and are often dependent ontemperature and synthesis factors. Pt—Co alloys crystallize instructures from primitive cubic to primitive tetragonal, dependent oncomposition. Clearly, symmetry variations are to be expected across amulti-component compositional range. Thus, the ternary alloys may beexpected to crystallize in a similar fashion to the previously describedstructures in a composition-dependent manner. For example, as the amountof cobalt added to platinum increases, the lattice of the resultingalloy may be expected to change from an fcc lattice to a tetragonalprimitive lattice. Symmetry changes (e.g., those associated with thechanges from a cubic face-centered structure to a primitive tetragonalstructure) may result in significant changes in the x-ray diffractionpattern. These changes may also be accompanied by more subtle changes inlattice parameters that may be indicative of the resulting changes inthe size of the respective metal constituents. For example, the12-coordinate metallic radii of platinum, cobalt, and manganese are 1.39Å, 1.25 Å, and 1.37 Å respectively, and as metals are substituted forplatinum, the average metal radius, and consequently the observedlattice parameter of a disordered alloy may be expected to contract orexpand accordingly. Thus, in the case of disordered alloys, the averageradius may be used as an indicator of lattice changes as a function ofstoichiometry, or alternatively, as an indicator of stoichiometry basedon observed diffraction patterns. However, while the average radii maybe useful as a general rule, experimental results are typically expectedto conform only in a general manner because local ordering, significantsize disparity between atoms, significant changes in symmetry, and otherfactors may produce results that are inconsistent with expectations.This may be particularly true in the case of ordered alloys where anincrease in the directionality of bonding may occur.

An interpretation of XRD analysis for the foregoing supported alloys isset forth below. Interpretation of XRD analyses can be subjective, andtherefore, the following conclusions are not intended to be limiting.The predicted change in the average radius for the Pt₄₇Mo₁₆Co₃₇ alloys(HFC 14 and 15) was a contraction of 3.8% versus platinum. The XRDdetermined contraction of HFC 14 was about 2.8%, slightly less thanpredicted. However, the onset of ordering, similar to PtCo wasindicated. As described above, the radius of manganese is larger thanthat of cobalt.

The predicted change in the average radius for the Pt₃₀Mn₂₈Co₄₂ alloys(HFC 61 and 62) was a decrease of 4.4% versus platinum. The XRD datashowed HFC 61 and 62 to be a mixed phase material containing both anordered alloy similar to PtCo (contraction of −1.5% vs. platinum) butwith larger lattice parameters and cobalt metal.

The predicted change in the average radius for the Pt₅₀Mn₁₁Co₃, alloys(HFC 126-129) was a contraction of 3.9% vs. platinum. The XRD determinedcontraction of HFC 126 was about 2.3%, again less than predicted.However, the onset of ordering similar to PtCo was indicated. Slightdifferences were noted between the 17.5 wt % materials (HFC 128 and 129)and the 31.5 wt % materials (HFC 126 and 127).

In view of the foregoing, for a particular electrocatalyst composition adetermination of the optimum conditions is preferred to produce thehighest activity for that particular composition. For example, thestarting materials used to synthesize the alloy may play a role in theactivity of the synthesized alloy. Specifically, using something otherthan a metal nitrate salt solution to supply the metal atoms may resultin different activities. Different methods of synthesis (e.g., chemicalprecipitation and freeze-drying impregnation) may result in differingactivities due to both differences in particle size and/or differencesin stoichiometric control. Heat treatment parameters such as atmosphere,time, temperature, etc. may also need to be optimized. This optimizationmay involve balancing competing phenomena. For example, increasing theheat treatment temperature is generally known to improve the reductionof a metal salt to a metal which typically increases activity; however,it also tends to increase the size of the electrocatalyst alloy particleand decrease surface area, which decreases electrocatalytic activity.

Definitions

Activity is defined as the maximum sustainable, or steady state, current(Amps) obtained from the electrocatalyst, when fabricated into anelectrode, at a given electric potential (Volts). Additionally, becauseof differences in the geometric area of electrodes, when comparingdifferent electrocatalysts, activity is often expressed in terms ofcurrent density (A/cm²).

An alloy is a mixture comprising two or more metals. An alloy may bedescribed as a solid solution in which the solute and solvent atoms (theterm solvent is applied to the metal that is in excess) are arranged atrandom, much in the same way as a liquid solution may be described. Ifsome solute atoms replace some of those of the solvent in the structureof the latter, the solid solution may be defined as a substitutionalsolid solution. Alternatively, an interstitial solid solution is formedif a smaller atom occupies the interstices between the larger atoms.Combinations of the two types are also possible. Furthermore, in certainsolid solutions, some level of regular arrangement may occur under theappropriate conditions resulting in a partial ordering that may bedescribed as a superstructure. These solid solutions may havecharacteristics that may be distinguishable through characterizationtechniques such as XRD. Significant changes in XRD may be apparent dueto changes in symmetry, if more complete ordering occurs such as thatwhich occurs between Pt metal and Pt₃Fe. Although the global arrangementof the atoms is extremely similar in both cases, the relationshipbetween the locations of the Pt and Fe atoms is now ordered and notrandom resulting in different diffraction patterns. Further, ahomogeneous alloy is a single compound comprising the constituentmetals. A heterogeneous alloy comprises an intimate mixture of crystalsof individual metals and/or metallic compounds (see, StructuralInorganic Chemistry, A. F. Wells, Oxford University Press, 5^(th)Edition, 1995, chapter 29). An alloy, as defined herein, is also meantto include materials which may comprise elements which are generallyconsidered to be non-metallic. For example, some alloys of the presentinvention may comprise oxygen in atomic, molecular, and/or metallicoxide form. Additionally, some alloys of the present invention maycomprise carbon in atomic, molecular, and/or metal carbide form. Ifpresent, the amount of oxygen and/or carbon in the alloy is typically atwhat is generally considered to be a low level (e.g., less than about 5weight percent), however higher concentrations are also possible (e.g.,up to about 10 weight percent).

It is to be understood that the above description is intended to beillustrative and not restrictive. Many embodiments will be apparent tothose of skill in the art upon reading the above description. The scopeof the invention should therefore be determined not with reference tothe above description alone, but should be determined with reference tothe claims and the full scope of equivalents to which such claims areentitled.

1. A ternary catalyst for use in oxidation or reduction reactions, theternary catalyst comprising platinum, manganese, and cobalt, with theconcentration of manganese being no greater than 20 atomic percent. 2.The ternary catalyst of claim 1 consisting essentially of platinum,manganese, and cobalt.
 3. The ternary catalyst of claim 1 wherein theconcentration of manganese is between about 5 and 15 atomic percent. 4.The ternary catalyst of claim 1 wherein the concentration of manganeseis between about 8 and 12 atomic percent.
 5. The ternary catalyst ofclaim 1 wherein the concentration of platinum is between about 10 andabout 70 atomic percent, and the concentration of cobalt is betweenabout 10 and about 70 atomic percent.
 6. The ternary catalyst of claim 1wherein the concentration of platinum is between about 20 and about 60atomic percent, and the concentration of cobalt is between about 20 andabout 60 atomic percent.
 7. The ternary catalyst of claim 1 wherein theconcentration of platinum is between about 30 and about 50 atomicpercent, and the concentration of cobalt is between about 30 and about50 atomic percent.
 8. The ternary catalyst of claim 1 wherein theconcentration of platinum is between about 40 and about 60 atomicpercent, the concentration of manganese is between about 10 and 20atomic percent, and the concentration of cobalt is between about 25 andabout 45 atomic percent.
 9. The ternary catalyst of claim 1 wherein theconcentration of platinum is between about 45 and about 55 atomicpercent, the concentration of manganese is between about 12 and 18atomic percent, and the concentration of cobalt is between about 30 toabout 40 atomic percent.
 10. The ternary catalyst of claim 1 wherein theternary catalyst is an alloy.
 11. A supported electrocatalyst powder foruse in electrochemical reactor devices, the supported electrocatalystpowder comprising the ternary catalyst of claim 1 and electricallyconductive support particles upon which the ternary catalyst isdispersed.
 12. The supported electrocatalyst powder of claim 11 whereinthe electrically conductive support particles are selected from thegroup consisting of inorganic supports and organic supports.
 13. Thesupported electrocatalyst powder of claim 12 wherein the electricallyconductive support particles are selected from the group consisting ofcarbon supports and electrically conductive polymer supports.
 14. A fuelcell electrode, the fuel cell electrode comprising electrocatalystparticles and an electrode substrate upon which the electrocatalystparticles are deposited, the electrocatalyst particles comprising theternary catalyst of claim
 1. 15. The fuel cell electrode of claim 14wherein the electrocatalyst particles comprise electrically conductivesupport particles upon which the ternary catalyst is dispersed.
 16. Thefuel cell electrode of claim 15 wherein the electrically conductivesupport particles are selected from the group consisting of carbonsupports and electrically conductive polymer supports.
 17. A fuel cellcomprising an anode, a cathode, a proton exchange membrane between theanode and the cathode, and the ternary catalyst of claim 1 for thecatalytic oxidation of a hydrogen-containing fuel or the catalyticreduction of oxygen.
 18. The fuel cell of claim 17 wherein the fuelconsists essentially of hydrogen.
 19. The fuel cell of claim 17 whereinthe fuel is a hydrocarbon-based fuel.
 20. The fuel cell of claim 17wherein the fuel comprises methanol.
 21. The fuel cell of claim 17wherein the ternary catalyst is on the surface of the proton exchangemembrane and in contact with the anode.
 22. The fuel cell of claim 17wherein the ternary catalyst is on the surface of the anode and incontact with the proton exchange membrane.
 23. The fuel cell of claim 17wherein the ternary catalyst is on the surface of the proton exchangemembrane and in contact with the cathode.
 24. The fuel cell of claim 17wherein the ternary catalyst is on the surface of the cathode and incontact with the proton exchange membrane.
 25. A method for theelectrochemical conversion of a hydrogen-containing fuel and oxygen toreaction products and electricity in a fuel cell comprising an anode, acathode, a proton exchange membrane therebetween, the ternary catalystof claim 1, and an electrically conductive external circuit connectingthe anode and cathode, the method comprising contacting thehydrogen-containing fuel or the oxygen and the ternary catalyst tocatalytically oxidize the hydrogen-containing fuel or catalyticallyreduce the oxygen.
 26. The method of claim 25 wherein thehydrogen-containing fuel consists essentially of hydrogen.
 27. Themethod of claim 25 wherein the hydrogen-containing fuel is ahydrocarbon-based fuel selected from the group consisting of saturatedhydrocarbons, garbage off-gas, oxygenated hydrocarbons, fossil fuels,and mixtures thereof.
 28. The method of claim 25 wherein thehydrogen-containing fuel is methanol.
 29. An unsupported ternarycatalyst layer on a surface of an electrolyte membrane or an electrode,said unsupported ternary catalyst layer comprising platinum, manganese,and cobalt.
 30. The unsupported ternary catalyst layer of claim 29wherein said layer has a thickness of about 10 to about 500 angstroms.31. The unsupported ternary catalyst layer of claim 29 wherein saidlayer has a thickness of about 20 to about 200 angstroms.
 32. Theunsupported ternary catalyst layer of claim 29 wherein said layer has athickness of about 40 to about 100 angstroms.
 33. The unsupportedternary catalyst layer of claim 29 wherein said layer has a surfaceconcentration of the unsupported ternary catalyst of less than about 5mg/cm².
 34. The unsupported ternary catalyst layer of claim 29 whereinsaid layer has a surface concentration of the unsupported ternarycatalyst of less than about 1 mg/cm².
 35. The unsupported ternarycatalyst layer of claim 29 wherein said layer has a surfaceconcentration of the unsupported ternary catalyst ranging from about 0.5mg/cm² to less than about 5 mg/cm².
 36. The unsupported ternary catalystlayer of claim 29 wherein said layer has a surface concentration of theunsupported ternary catalyst ranging from about 0.1 mg/cm² to less thanabout 1 mg/cm².
 37. The unsupported catalyst layer of claim 29 whereinsaid unsupported ternary catalyst layer has a manganese concentrationthat is no greater than 20 atomic percent.